In the last 30 years, interracial marriage in the United States has more than doubled, from 6.7 percent in 1980 to just over 15 percent in 2010, according to a 2012 Pew Research Center report. Yet given that minorities comprise 37 percent of the U.S. population — and will exceed 50 percent by 2043, according to the U.S. Census Bureau — mixed-race marriage remains relatively rare. The vast majority of Americans still settle down with someone who looks a lot like them. What's more, a new study shows that a preponderance of the same-race marriages in America are the result of personal preference and not just environmental factors, like living in a homogenous neighborhood.

The research, conducted by Neil Malhotra, a political scientist at Stanford Graduate School of Business, looked at racial preferences expressed at one of the earliest possible stages of a relationship: when a dating site user scrolling through headshots and brief bios of potential mates chooses which ones to click on for more information.

Malhotra and his colleagues — Ashton Anderson, a Stanford doctoral candidate in computer science, and Greg Huber, a political scientist at Yale — teamed up with scientists at Microsoft Research to analyze a vast store of anonymous profiles on Yahoo's now defunct online dating site, Yahoo Personals. They sought to examine how strongly a user's stated preference for the race of a potential partner matched up not only with the race of the potential suitors whose photos he or she clicked on, but also with his or her political ideology.

They found that even when respondents said they couldn't care less about a date's race, their actions spoke louder: The majority overwhelmingly viewed profiles of same-race prospects. "We find not only that a large proportion of our population states a same-race preference and acts on it, but that even individuals who state that they do not have a preference act as if they do."

The researchers focused on heterosexuals who identified themselves as either white or black. Their sampling was socioeconomically and geographically diverse, and unusually large, drawing on data from the activity logs of more than 250,000 people who had used Yahoo's online dating service for two months in 2009. On their demographic profiles, the subjects had noted whether they considered a partner of the same race a "must-have," a "nice-to-have," or unimportant, and rated their political views on a five-point scale, from very liberal to very conservative. They also controlled for factors such as income, education level, religion, and drinking habits, among many others. Though the racial makeup of the data group roughly mirrored that of the U.S. population, the gender split skewed heavily male, to 75 percent, reflecting the relatively high proportion of men typical of online dating sites.

Liberals were less likely than conservatives to both state a preference for a same-race preference and act on that preference. That does not mean liberals are more colorblind; even liberals who claimed they had no racial preferences exhibited them through the profiles they opened.

The study showed, however, that ideology has little bearing on a person's "willingness to express same-race preferences relative to acting on them," Malhotra wrote.

"Conservatives are more likely than liberals to state a same-race preference, but for any given level of stated preferences, both conservatives and liberals show a similar propensity to act," the research paper says.

In other words, liberals were no more likely than conservatives to exhibit a discrepancy between their stated and revealed preferences. Going into the study, Malhotra says, some speculated that liberals might claim to be less concerned about race than they actually were because "they wanted to signal to potential dates and themselves that they were open-minded and free-thinking people."

What are the implications at a policy level? Even though a more integrated society strengthens communities and creates opportunities, racial homogamy remains the norm. Same-race marriage derives partly from personal preference, and it can't be changed merely "by creating more opportunities for individuals of different races to interact," write the researchers.

Racial homogamy persists even among people who say race doesn't matter, and it's especially prevalent among political conservatives. "We seem to be on a trend toward more interracial marriages and therefore more mixed-race children," says Malhotra. "But our findings raise concerns that these patterns are mainly manifesting themselves among liberal enclaves in America, further heightening the cultural divide between left and right." Policymakers seeking to reduce political polarization clearly have their work cut out for them.

Neil Malhotra is an associate professor of political economy at Stanford Graduate School of Business.